Gpi16 (SPBC1604.15) is the fission-yeast member of the PIG-T/Gpi16 family and a predicted non-catalytic subunit of the endoplasmic-reticulum GPI-anchor transamidase, the multisubunit complex (Gpi8/PIG-K, Gaa1/GPAA1, Gpi16/PIG-T, Gpi17/PIG-S and Gab1/PIG-U) that cleaves the C-terminal GPI-attachment signal of precursor proteins and joins the newly exposed C terminus to a preassembled GPI anchor. The 545-residue type-I membrane protein has an N-terminal signal peptide, a large lumenal PIG-T domain, a single C-terminal transmembrane helix and a short cytosolic tail. By orthology to budding-yeast Gpi16 and mammalian PIG-T, it is expected to stabilise the catalytic Gpi8 subunit and the assembled complex and to help hold substrate proteins, whereas the cysteine-protease chemistry of the reaction belongs to Gpi8. The protein localises to the ER in fission yeast; its function there has not been tested directly in this species.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: No molecular activity has been demonstrated for Gpi16 itself; the PANTHER/PAINT node that supplies its complex and process terms carries no molecular-function assertion. Reason: Retain the ND root annotation. Complex membership and participation in GPI attachment do not establish an activity the subunit performs on its own: in the human structure the catalytic component is PIGK, and the PAINT node PTN000311103 (taxon Eukaryota) propagates only the complex and the attachment process, not a transamidase or peptidase activity. A subunit-level contributes_to GPI-anchor transamidase activity is not carried by human PIGT in GOA either, so none is proposed here. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:35165458 The PIGK subunit functions as the catalytic component |
| GO:0005783 endoplasmic reticulum | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | MODIFY | Summary: The genome-wide ORFeome GFP-localisation screen placed Gpi16 in the ER; this is the only direct experimental observation for the fission-yeast protein. Reason: The localisation is correct but more general than the evidence supports. Gpi16 is a single-pass type-I membrane protein (signal peptide, lumenal domain, C-terminal transmembrane helix), as the budding-yeast ortholog was shown biochemically to be, so the specific location is the ER membrane, a term the gene already carries. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- SUBCELLULAR LOCATION: Endoplasmic reticulum membrane PMID:11598210 Its bulk resides on the lumenal side of the ER, and it has a single C-terminal transmembrane domain |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Gpi16 is an integral ER-membrane protein. Reason: The UniProt subcellular-location statement rests on the fission-yeast ORFeome observation (ECO:0000269, PMID:16823372), and the predicted type-I topology matches the experimentally determined topology of budding-yeast Gpi16, a member of the same PANTHER subfamily (PTHR12959:SF11). Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- SUBCELLULAR LOCATION: Endoplasmic reticulum membrane PMID:11598210 Its bulk resides on the lumenal side of the ER, and it has a single C-terminal transmembrane domain |
| GO:0005789 endoplasmic reticulum membrane | NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s... | ACCEPT | Summary: Gpi16 is an integral ER-membrane protein. Reason: Consistent with the IEA row for the same term. The ComplexPortal NAS row cites a fungal GPI-signal prediction paper that discusses the fungal transamidase complex but does not test Gpi16 localisation; the location itself is supported by the fission-yeast ER observation, the type-I topology and the ortholog data. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- SUBCELLULAR LOCATION: Endoplasmic reticulum membrane PMID:11598210 Its bulk resides on the lumenal side of the ER, and it has a single C-terminal transmembrane domain |
| GO:0006506 GPI anchor biosynthetic process | IEA GO_REF:0000041 | MODIFY | Summary: The UniPathway mapping places Gpi16 in synthesis of the GPI lipid anchor, but the transamidase acts after the anchor is complete, attaching it to protein. Reason: GO:0006506 covers formation of the GPI glycolipid itself. GO:0016255 attachment of GPI anchor to protein is not a descendant of it (it sits under GO:0180046 GPI anchored protein biosynthesis), so this is a lateral correction to the step the transamidase actually performs. In budding yeast, depleting Gpi16 leaves the complete GPI lipid accumulated rather than blocking its synthesis, which places the subunit after anchor assembly. The gene already carries GO:0016255. Proposed replacements: attachment of GPI anchor to protein Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 Depletion of Gpi16p results in the accumulation of the complete GPI lipid CP2 and of unprocessed GPI precursor proteins. |
| GO:0016255 attachment of GPI anchor to protein | IBA GO_REF:0000033 | ACCEPT | Summary: Gpi16 takes part in attaching GPI anchors to proteins as a subunit of the transamidase. Reason: The PAINT IBD is placed at PTN000311103 (taxon Eukaryota) in PTHR12959, with experimental descendants in budding yeast, fly, mouse and human. Gpi16 is in the same subfamily (SF11) as budding-yeast Gpi16 and human PIGT, and has the full-length PIG-T domain and the type-I membrane architecture, so nothing suggests the function was lost. Budding-yeast Gpi16 is required for processing of GPI precursor proteins, and as a structural subunit of the complex it does part of the work of the attachment reaction rather than merely being needed for it. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11598210 Depletion of Gpi16p results in the accumulation of the complete GPI lipid CP2 and of unprocessed GPI precursor proteins. PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. |
| GO:0016255 attachment of GPI anchor to protein | IEA GO_REF:0000002 | ACCEPT | Summary: Gpi16 takes part in attaching GPI anchors to proteins as a subunit of the transamidase. Reason: Consistent with the IBA row. The InterPro PIG-T signature (IPR007245) covers the whole family, and its members characterised so far are all transamidase subunits. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. |
| GO:0016255 attachment of GPI anchor to protein | NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s... | ACCEPT | Summary: Gpi16 takes part in attaching GPI anchors to proteins as a subunit of the transamidase. Reason: Consistent with the IBA row. The cited paper is about predicting fungal GPI-attachment signals and refers to the fungal transamidase complex only as context; the annotation is supported by orthology to the experimentally characterised budding-yeast and mammalian subunits. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. |
| GO:0031505 fungal-type cell wall organization | NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s... | MARK AS OVER ANNOTATED | Summary: Many fission-yeast GPI-anchored proteins are cell-wall proteins, so GPI attachment is upstream of cell-wall assembly; Gpi16 itself acts in the ER and does none of the work of organising the wall. Reason: The ComplexPortal NAS row propagates a downstream consequence of the complex's output to every subunit. Gpi16 contributes to attaching anchors to precursor proteins in the ER; organising the cell wall is carried out by the GPI-anchored glycoproteins and cell-wall enzymes that are processed later. Being needed upstream of a process is not taking part in it. No fission-yeast gpi16 mutant cell-wall phenotype has been reported, and the cited paper predicts GPI-attachment signals rather than testing Gpi16. The annotation is not false at the level of the complex, so it is flagged rather than removed. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. file:SCHPO/gpi16/gpi16-deep-research-falcon.md no retrieved study directly connected an O94380 mutation to those phenotypes |
| GO:0042765 GPI-anchor transamidase complex | IBA GO_REF:0000033 | ACCEPT | Summary: Gpi16 is the PIG-T subunit of the GPI-anchor transamidase complex. Reason: The PAINT IBD at PTN000311103 (Eukaryota) is grounded in affinity purification of budding-yeast Gpi16 with Gaa1 and Gpi8 and in purification of the mammalian complex with PIG-T. Fission-yeast Gpi16 shares PANTHER subfamily SF11 with both, and the budding-yeast authors already noted that Gpi16 homologues occur in all eukaryotes. This is the core cellular-component annotation. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. PMID:11598210 Homologues of Gpi16p are found in all eucaryotes. |
| GO:0042765 GPI-anchor transamidase complex | IEA GO_REF:0000002 | ACCEPT | Summary: Gpi16 is the PIG-T subunit of the GPI-anchor transamidase complex. Reason: Consistent with the IBA row; PIG-T family membership (IPR007245) implies transamidase-complex membership for full-length members. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. |
| GO:0042765 GPI-anchor transamidase complex | NAS PMID:15003443 A sensitive predictor for potential GPI lipid modification s... | ACCEPT | Summary: Gpi16 is the PIG-T subunit of the GPI-anchor transamidase complex. Reason: Consistent with the IBA row. The ComplexPortal fission-yeast complex entry is a curated inference from orthology; the cited paper does not purify the fission-yeast complex. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt CC -!- FUNCTION: Component of the GPI transamidase complex. Involved in CC transfer of GPI to proteins (By similarity). {ECO:0000250}. PMID:11598210 These complexes can be affinity purified and are shown to consist of Gaa1p, Gpi8p, and Gpi16p (YHR188c). PMID:11483512 PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. |
| GO:0098553 lumenal side of endoplasmic reticulum membrane | IC GO_REF:0000111 | ACCEPT | Summary: The bulk of Gpi16, its PIG-T domain, faces the ER lumen, where the transamidase acts on its substrates. Reason: PomBase inferred this from complex membership. The predicted topology (signal peptide 1-22, lumenal domain 23-493, transmembrane helix 494-514) agrees with the experimentally determined topology of budding-yeast Gpi16, whose bulk resides on the lumenal side of the ER. Supporting Evidence: file:SCHPO/gpi16/gpi16-uniprot.txt FT TOPO_DOM 23..493 FT /note="Lumenal" FT /evidence="ECO:0000255" PMID:11598210 Its bulk resides on the lumenal side of the ER, and it has a single C-terminal transmembrane domain |
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Download this section (compressed HTML)Q: Does fission-yeast Gpi16 form an intermolecular disulfide with Gpi8 as mammalian PIG-T does with PIGK, or does it behave like budding-yeast Gpi16, which reportedly does not?
Q: Should GO represent the accessory-subunit role of PIG-T family proteins with a molecular-function term (for example a contributes_to transamidase activity, or a protein-stabilising activity), given that PAINT and human PIGT currently carry none?
Experiment: Construct a conditional (shut-off) gpi16 allele in fission yeast and test for accumulation of the complete GPI lipid and of unprocessed GPI precursor proteins, as was done for budding-yeast Gpi16, together with tests of cell-wall integrity.
Type: genetic depletion with lipid and protein biochemistry
Experiment: Purify tagged fission-yeast Gpi8 from digitonin extracts and identify co-purifying subunits by mass spectrometry, to confirm that Gpi16 is part of the native complex in this species.
Type: affinity purification and mass spectrometry
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